The present invention relates to a cold-cathode power switching device of field emission type, and more particularly to a cold-cathode array substrate which has a plurality of cold-cathode modules of field emission type and which is suitable for use in a large-current, high-voltage power switching device, and also to a method of manufacturing a cold-cathode power switching device of field emission type.
Hitherto, cold-cathode devices of field emission type have been developed for use mainly in displays. A cold-cathode device of this type is used in a display, as a source of electron beams for illuminating the fluorescent screen of the display. Therefore, the current supplied to the device is small, and the voltage applied thereto is only 1 kV or less.
In recent years, it has been proposed that the cold-cathode device of field emission type be used as a power switching device. A power switching device needs to operate for current ranging from a few tens of amperes to several thousands of amperes and for voltages ranging from a few kilovolts to several hundreds of kilovolts.
To allow passage of a large current, a cold-cathode device of field emission type must have a relatively large device area. In order to facilitate emission of electrons, however, it is desirable that the cone-shaped cathode of the cold-cathode device have an extremely sharp tip. It is very difficult for the cold-cathode device, which is manufactured by micro-structure process, not only to have a large device area but also to achieve uniform electron emission over the large device area. This is why the cold-cathode device of field emission type, hitherto made, is disadvantageous in terms of reliability and manufacturing yield.
When a high voltage of 1 kV or more is applied to the conventional cold-cathode device of field emission type, discharge takes place at an uneven part of the device, possibly resulting in malfunction or voltage breakdown of the cold-cathode device. Such an uneven part is formed, particularly in a cold-cathode device which is an active device having gate electrodes for supplying control signals. To prevent discharge, it is necessary to arrange the gate electrodes and gate wiring such an uneven part is not formed in the cold-cathode device.
There is a demand for a large-current, high-voltage cold-cathode power device of field emission type. To meet the demand, a multi-module device may be used which has a number of cold-cathode modules. The multi-module device needs complex gate wiring to connect the gate electrodes of the many cold-cathode modules. The complex gate wiring is likely to form an uneven part in the multi-module device, at which discharge may occur.
It is extremely difficult to use a multi-module cold-cathode device of field emission type as a power switching device. Thus, the demand for a large-current, high-voltage cold-cathode power device of field emission type has not been satisfied.
Cold-cathode devices of field emission type, which have a cold-cathode array substrate, may have high-speed response, good anti-radiation property and high heat resistance and which may operate for large current and high voltages. Researches have, therefore, been made of the cold-cathode devices with a cold-cathode array substrate, which have these advantageous features.
Research and development of a cold-cathode device of field-emission type was started by K. R. Shoulders et al. at Stanford Research Institute (SRI), who proposed a tunnel effect vacuum triode in their thesis "Microelectronics using electron-beam-activated machining techniques," Advances in Computers, Voltage. 2, pp. 135-293, 1961. This field of art came to attract attention of many researchers when C. A. Spindt of SRI published a report on cold cathodes having a thin film (see J. Appl. Phys. 39, p. 3504, 1968).
A cold-cathode device of field emission type comprises an emitter electrode, an anode electrode, a cone-shaped emitter, and a gate electrode. When a high voltage is applied between the emitter electrode and the anode electrode, the emitter emits electrons, whereby main current flows. The main current is controlled by supplying a control signal to the gate.
The cone-shaped emitter is a miniature metal emitter. How the miniature metal emitter is made, along with the gate, by so-called "Spindt method" will be explained, with reference to FIGS. 1A to 1C. The Spindt method is most widely used at present. In this method, rotational grazing vapor deposition and aluminum (Al) sacrifice layer etching are performed.
As shown in FIG. 1A, a gate insulating film 6 is formed on a silicon substrate 1a. A gate layer 4a, which is a thin metal film, is formed on the gate insulating film 6. The gate insulating film 6 is etched by using the gate layer 4a as a mask. An opening is thereby made in the gate insulating film 6.
Next, as shown in FIG. 1B, Al is deposited on the gate layer 4a by effecting rotational grazing vapor-deposition at a small grazing angle .phi.. An Al sacrifice layer 31 is thereby formed on the gate insulating film 6. Since the grazing angle is small as shown in FIG. 1B, Al is deposited on the gate layer 4a only, not on the silicon substrate 1a at all.
Then, as shown in FIG. 1C, molybdenum (Mo) vapor is applied in vertical direction onto the silicon substrate 1a through the opening made in the film 6. Mo is thereby deposited on the substrate 1a, forming an emitter 26. The emitter 26 is shaped like an acute cone, because the opening made in the Al sacrifice layer 31 gradually narrows as the deposition of Mo proceeds on the Al sacrifice layer 31.
The method of forming a miniature metal emitter, which Gray et al. has proposed, will be described with reference to FIGS. 2A to 2C.
First, as shown in FIG. 2A, an SiO.sub.2 etching mask 32 is formed and patterned on a silicon substrate 1a. As shown in FIG. 2B, anisotropic wet etching solution is applied, thereby etching the silicon substrate 1a along the crystal plane. The silicon substrate 1a is thereby etched at its upper surface, except that part which is located beneath the SiO.sub.2 etching mask 32. As the anisotropic etching further proceeds, that part of the substrate 1a assumes a shape like a pyramid, and the SiO.sub.2 etching mask 32 is removed from the silicon substrate 1a. As a result, a pyramid-shaped miniature silicon emitter 1b is formed, which protrudes from the silicon substrate 1a.
Next, a gate insulating film 6 is deposited on the silicon substrate 1a, and a gate layer 4a is deposited on the gate insulating film 6. As shown in FIG. 2C, an opening is made in that part of the gate layer 4a which is located above the miniature silicon emitter 1b. Selective etching is performed on the gate insulating film 6 by using the gate layer 4a as a mask. An opening is thereby formed in the gate insulating film 6 exposing the miniature silicon emitter 1b.
The Spindt method and the Gray et al. method, described above, include microstructure process. It is therefore very difficult to form a number of miniature emitters on the silicon substrate, with a sufficiently high yield. No practical assembling methods that can arrange many cold-cathode tips in the form of an array.
A number of cold-cathode tips may be formed on a cold-cathode array substrate for use in a power device by these method. If any one of the miniature emitters is short-circuited with the gate layer, however, the cold-cathode array substrate will become useless in its entirety. This reduces the manufacturing yield of the cold-cathode array substrate.
The cold-cathode array substrate has projections protruding from its periphery. Like the miniature emitters, the projections are likely to emit electrons. If the projections emit electrons, a leakage current is generated, eventually degrading the voltage resistance of the power device comprising the cold-cathode array substrate. It should be noted that the gate layer cannot control the leakage current.
The silicon substrate 1a of Gray acts as the series resistance on the main current flowing in the miniature emitters 1b formed by etching the surface of the substrate 1a. This decreases the operating speed of the power device. Should the temperature of the power device rise while the device is operating, the tip of every miniature silicon emitter 1b would degrade and the service time of the power device will become short.
The Gray et al. method is less complicated than the Spindt method (FIGS. 1A to 1C) in which miniature metal emitters 26 are formed by depositing Mo on the silicon substrate 1a. However, the tips of the emitters 1b are likely to degrade as the temperature of the silicon substrate 1a of the power device rises, ultimately shortening the service time of the power device.
The tips of the miniature metal emitters 26 made of Mo and formed by the Spindt method also degrade as the power device in which the emitters 26 are provided generates much heat while operating. This is inevitable because the substrate 1a is made of silicon.
As described above, the current density in any conventional cold-cathode device of field emission type cannot be increased, because much heat will be generated in the substrate if the current density is high. The cold-cathode device cannot be modified to operate for large current and high voltages. In view of this, the possibility is slim that the conventional cold-cathode devices are used as switching devices.